Cadmium and mercury are extremely nephrotoxic and the understanding of their mechanism of action is one essential step in preventing or reversing their detrimental effect on the kidney. This study is designed to provide insights into the action of heavy metals on critically involved in the urinary concentrating process. Using as a model isolated plasma membranes of the rectal gland of the shark, which are an abundant source for this cotransporter, their mechanism of action will be first characterized kinetically with regard tot he ion transport and bumetanide binding sites. Then the amino acid side chains involved in this interaction between heavy metals and the specific reagents with that of the toxins. Finally, the amino acids carrying these heavy metal- sensitive side chains will be localized at the molecular level. These studies will contribute substantially to the understanding of the action of heavy metals at the transporter protein level and thereby provide important information on potential target sites, putative receptors and possible binding motifs of heavy metals and their chemical nature.